Adequate dietary intake of thiol amino acids has been implicated as an important factor in the ability to withstand chemical and environmental insults that elicit oxidative stress. Without adequate free cysteine, the ability to restore glutathione that is lost due to oxidation is compromised by inadequate glutathione synthesis where sensitive target cells and tissues are damaged or killed due to the consequences of unresolved oxidative stress. The mechanisms of some important human diseases and adverse health conditions (e.g., HIV, malnutrition, inflammation) have been directly linked to low systemic glutathione, secondary to decreases in cysteine availability. Similar consequences of depleted and/or oxidized glutathione, leading to exacerbation of embryotoxicity and increased frequency and severity of teratogenic lesions, have been reported. Several human teratogens are known to elicit oxidative stress, although relatively little is known about the mechanisms that regulate antioxidant activity in the conceptus. Demonstrated differences in the rates of new glutathione synthesis in the developing embryo and its extra-embryonic membranes suggest that the availability of free cysteine, as the rate-limiting precursor for new glutathione synthesis, might be important for the determination of selectivity or resistance to chemical embryotoxicants. The investigators hypothesize that the availability of free cysteine in the embryo and visceral yolk sac of the post-implantation embryo determines the extent to which the embryo is able to maintain adequate glutathione concentrations and respond to chemically-induced oxidative stress by synthesizing new glutathione for restoration of normal redox status. Preliminary experiments with chemical embryotoxins, such as the pesticides lindane and aminocarb, show selective decreases in free cysteine concentrations prior to observed depletion of glutathione and onset of toxicity. Interspecies comparisons between the rat and rabbit show significant reductions in tissue glutathione and cysteine in the rabbit that may help explain the greater sensitivity of the rabbit to teratogens that produce oxidative stress such as thalidomide. The investigators propose to compare glutathione/glutathione disulfide/cysteine status and rates of new glutathione synthesis between rat and rabbit conceptual tissues. They will assess the effects of lindane and aminocarb on these endpoints and determine whether mechanistic relationships exist between a species' or tissue's ability to obtain cysteine and synthesize new glutathione and specific cell sensitivity or resistance to toxic chemicals. Whole animal, whole embryo culture and cell cultures (micromass limb/midbrain and spinal neural crest) will used to assess the role of cysteine in embryoprotection. In vitro and in vivo experiments will determine whether dietary restriction of cysteine differentially exacerbates toxicity in the two species and/or whether dietary or direct supplementation of cysteine is adequate to protect the embryo from chemical insult.